1. Overview of sh_gamit / sh_glred processing
Create /tables, /rinex, /gsoln directories, run sh_setup
- in /tables, copy/edit control files
- in /rinex, copy/link RINEX files
- in /gsoln, copy/edit command files
sh_gamit -expt [expt-name] -s [yr] [start-doy] [stop-doy]
sh_glred -expt [expt-name] -s [yr] [start-doy] [yr] [stop-doy] -opt H G E
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Introduction to GAMIT/GLOBK

2. Introduction to GAMIT/GLOBK
Directory Structure
Top level: global tables and survey directories
Within each survey directory:
/tables /rinex /igs /gfiles /brdc /gsoln /glbf
/day1 /day2 …
Tables are linked from day directories to experiment /tables and then to gg/tables
GAMIT processing occurs in the day directories
GLOBK processing occurs in /gsoln
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Introduction to GAMIT/GLOBK

3. Functions of GAMIT Programs
makexp and makex prepare the data
fixdrv prepares the batch control files
arc integrates GPS satellite orbits
model calculates theoretical (modeled) phase and partial derivatives of phase w.r.t. parameters
autcln cleans GPS data and repairs cycle slips
solve estimates parameters via least squares
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Introduction to GAMIT/GLOBK

4. Steps in the standard GAMIT batch sequence
arc, model, autcln, solve for initial solution
5-minute sampling, no ambiguity resolution (GCR only)
update lfile. for coordinates adjusted > 30 cm
--> autcln.prefit.sum, qexptp.ddd
model, autcln, solve for final solution
2-minute sampling, ambiguity resolution
--> autcln.post.sum, qexpta.ddd ( GCR, GCX, GLR, GLX )
Final solution repeated if nrms reduced by > 30% from initial solution
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Introduction to GAMIT/GLOBK

5. Files you need to worry about
RINEX files – local plus list in sites.defaults
Control files
process.defaults – minor edits for each survey
sites.defaults – sites to include/omit; source of metadata
sestbl. – unchanged for most processing
sittbl. – sites constrained for ambiguity resolution
globk_comb.cmd – apr_neu, apr_svs, apr_wob, apr_ut1
glorg_comb.cmd – apr_file, pos_org, stab_site
A priori coordinates ( apr-file, l-file )
Meta-data (station.info)
Differential code biases (dcb.dat) – download current values 1/month
Satellite characteristics (svnav.dat) – download current w/ each new launch
Templates for these files are in the study notes. pp 5-11, and the individual entries are discussed in Chapter 2 of the Introduction to GAMIT/GLOBK and Chapter 3 of the GAMIT Reference Manual.
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Introduction to GAMIT/GLOBK

6. Files provided or created automatically
Satellite orbits
IGS sp3-files (tabular) or g-files (ICs for GAMIT)
ARC integrates to get t-files (tabular)
Earth Orientation Parameters ( ut1., wob.) - downloaded if needed for current day
Leap-second file -- linked to gg/tables (update ~yearly)
Satellite clock (j-) files – from RINEX navigation (brdc) file
Rcvr/ant characteristics (rcvant.dat, hi.dat) – linked to gg/tables
Differential code biases (dcb.dat)—update ~monthly
Antenna phase center models (antmod.dat) – linked to gg/tables
Luni-solar ephemerides and nutation (soltab., luntab., nutabl.) linked to gg/tables (need to update yearly)
Ocean tide grid (optional) – linked to gg/tables
Atmospheric loading grid (optional) – need to update yearly
Mapping function grid (optional) – need to update yearly
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Introduction to GAMIT/GLOBK

7. Options for metadata (station.info)
Pre-prepared station.info (make_stnfo, sh_upd_stnfo)
Must set –xstinfo in sites.defaults
RINEX headers (sh_gamit default)
Update station.info unless an entry already exists for the day being processed or stinf_unique is set to -u in process.defaults and entry has not changed
Can be used with non-standard receiver and antenna names specified in guess_rcvant.dat
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Introduction to GAMIT/GLOBK

8. A priori coordinates (sh_gamit)
Create l-file in day directory by merging existing lfile. and apr_file from ../tables (apr_file has priority)
If site not found in l-file
Use RINEX header coordinates (use_rxc=Y) or
Use pseudorange data in RINEX file to estimate point position or differential position relative to a site in sites.defaults (use_rxc=N, default)
During the sh_gamit run, the coordinates are updated (and copied to ../tables/lfile.) if they are in error by > 30 cm
Why do we do have this scheme? Need to maintain get and maintain coordinates < 0.3 m
GAMIT runs from L-file, either the older spherical format or the newer Cartesian (GLOBK apr) format. The L-file is updated after each solve run with the new estimates if the adjustments exceed the ‘Update tolereance’ (usually 0.3 m) in the sestbl. In sh_gamit processing, these values are carried forward to the next day except that at the beginning of each day, the coordinates in the L-file for any site appearing in the ‘apr_file’ (named in process.defaults), which usually includes ITRF sites and any regional sites for which previous processing has achieved precise coordinates.
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Introduction to GAMIT/GLOBK

9. Introduction to GAMIT/GLOBK
Ambiquity resolution
(L2-L1) resolved by autcln and passed to solve in the n-file ( LC_AUTCLN )
weak dependence on geometry
need current dcb.dat file
use LC_HELP for old data or if problems (default max distance is 500 km)
Narrow-lane (L1) resolved by solve
strong dependence on phase noise and models
constraints on good apriori coordinates may help
Confusing topic without knowing theory. Come back to later in the afternoon if requested.
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Introduction to GAMIT/GLOBK

10. sh_gamit_ddd.summary (email)
Contents (Yellow is output):
Input options -d expt ncar -pres ELEV -yrext -netext a
Processing GPS week Raw 2
/data51/tah/SENH02/glob02/suomi/2002_031a
Disk Usage: Free Mbyte. Used 15%
Summary Statistics ( from autcln )
Number of stations used 4 Total xfiles 4
Postfit RMS rms, to and by satellite
RMS IT Site All …
RMS 20 ALL …
Best and Worst two sites:
RMS 20 TMGO …
RMS 20 SA …
RMS 20 PLTC …
RMS 20 SA …
See also the example on p. 4 of the WS Notes.
Two parts: autcln summary and q-file.
This slide shows the top part of the summary file, comprising statistics extracted from autcln
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Introduction to GAMIT/GLOBK

11. sh_gamit_ddd.summary (email)
Solution statistics from solve
Double difference statistics
Prefit nrms: E Postfit nrms: E+00 Constrained free
Prefit nrms: E Postfit nrms: E+00 Constrained fixed
Prefit nrms: E Postfit nrms: E+00 Loose free
Prefit nrms: E Postfit nrms: E+00 Loose fixed
Number of double differences:
Numbers of WL and NL biases Perscent fixed 95% WL 85% NL
Any large adjustments to positions (>0.3 m)
Things to note:
Number of stations matches expectation
AUTCLN RMS 3-6 mm
No stations with RMS = 0 (no postfit coordinates)
SOLVE Postfit nrms ~0.2 for constrained and loose solutions
“Most” ambiguities resolved
The statistics in the bottom part of the summary have been extracted from the q-file.
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Introduction to GAMIT/GLOBK

12. Introduction to GAMIT/GLOBK
Phase Residual Plots
Set with -pres elev in sh_gamit command line (requires GMT)
Postscript files in day directory, by default converted to gif in /gifs directory and then erased
Use to assess multipath, water vapor, and antenna phase center model
The sky plots produced using the –pres ELEV command-line option of sh_gamit (or separately with sh_sky_gifs), are the most accessible source of detailed knowledge about conditions of session: multipath, atmosphere, receiver performance.
“Sky plot” Phase vs elevation angle
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Introduction to GAMIT/GLOBK

13. Introduction to GAMIT/GLOBK
High residuals in the same place at different times suggest mulitpath
High residuals appearing in a given place only at one time suggest water vapor
Later this afternoon when we talk about height, we’ll look at what you can learn in detail from these plots
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Introduction to GAMIT/GLOBK

14. Phase vs elevation angle
Normal pattern: bands are high-frequency multipath; red is smoothing of individual values, showing no strong systematics
Bad pattern: systematic signature of smoothed values indicates a poor model of the antenna phase pattern (perhaps a misidentified antenna in station.info)
Green lines show the elevation-dependent noise model shown at top and used to reweight the data in solve
Large excursion is below 10 deg but still systematic above 10 deg
THIS IS LAST SLIDE for presentation.
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Introduction to GAMIT/GLOBK

15. Introduction to GAMIT/GLOBK
What can go wrong?
Site missing (not listed)
- no RINEX data within session span: check RINEX file and/or makex.expt.infor
- too few data, x-file too small and not used: check RINEX file size, change minxf in process.defaults
Site in solution but no data or adjustment
- a priori coordinates > 10 m off: check range rms in autcln.prefit.sum, run sh_rx2apr differentially for several RINEX files
- bad receiver: examine RINEX files or initial c-files with cview
Q-file nrms > 0.2
- solution over-constrained: check GCX vs GLX nrms, rerun with only one site constrained
Next few slides look at each of these problems.
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Introduction to GAMIT/GLOBK

16. Problems with a priori coordinates
Need to be good to < 10 m to get through autcln
Safest source is a previous solution or a pseudorange solution using svpos/svdiff (sh_rx2apr)
Range rms and bias flags added from autcln summary file are useful check
Convergence is 1:100 to 1:1000 (1 m error in apr can lead to 1-10 mm error in adjustment)—hence automatic update of L-file for GAMIT 2nd solution
Watch for repeated updates in summary as a sign of bad data
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Introduction to GAMIT/GLOBK

17. Constraining the GAMIT solution
Minimal (single-station) constraint is all that’s needed for ambiguity resolution, but sittbl can list several to assure one
Orbits can be fixed or tightly constrained (.005 ppm) for IGS orbits since at least 1996
Look for good (~0.2) loose (GLR/GLX) nrms but elevated constrained nrms (GCR/GCX) as indication of an over-constrained solution
If using GLOBK for final solution, you can constrain the frame more rigorously there, so for GAMIT, just
Provide a minimal constraint to assess data quality.: low nrms, small adjustments to all parameters.
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Introduction to GAMIT/GLOBK

18. Introduction to GAMIT/GLOBK
More Subtle Problems
Site with high rms in autcln.post.sum
- high multipathing or water vapor: check sky plots of phase
- bad receiver: examine RINEX files or initial c-files with cview
Phase vs elevation angle plot large and systematic
- misidentified antenna (wrong PCV model)
- coupling between antenna and mount (discussed during height section)
GAMIT results within normal range but time series shows outlier
- change in multipath (water, objects) or water vapor
- snow on antenna
- incorrect ambiguity resolution (east component except for high latitudes)
These are somewhat overlapping, both with the three major problems discussed on the previous slides, and with each other. They provide a conceptual structure with which to look at the plotting tools available for diagnostics.
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Introduction to GAMIT/GLOBK

19. Introduction to GAMIT/GLOBK
Understanding
outliers
Autcln rms
Day mm
Day mm
Time series (from sh_glred) showing an outlier on day Why?
Probably troposphere since in height and we don’t expect multipth to change (though it can)
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Introduction to GAMIT/GLOBK

20. Introduction to GAMIT/GLOBK
ALBH 2003 Day 201
ALBH 2003 Day 202
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Introduction to GAMIT/GLOBK

21. Introduction to GAMIT/GLOBK
The atmospheric delay can vary by decimeters, and though the surface pressure provides an adequate calibration of the the gases in hydrostatic equilibrium, there is no reliable and economical way to remove the contribution of water vapor. So we estimate that portion, parameterized by a delay at the zenith and a mapping function that is reliable to 15 degree elevation angle, the usual lower limit for GPS observations. This slide shows the estimates at 2-hr intervals for two stations used in our analysis of PNW data from last summer.
GPS adjustments to atmospheric zenith delay for 29 June, 2003; southern Vancouver Island (ALBH) and northern coastal California (ALEN). Estimates at 2-hr intervals.
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Introduction to GAMIT/GLOBK